This invention pertains to electrical machines and more particularly to an airgap armature for use in brushless rotary electrical machines that convert between electrical and mechanical energy. The airgap armature provides for increased fill of windings in the active region while minimizing the airgap thickness. The airgap armature further provides high efficiency and allows for higher power energy conversion with reduced inductance, increased structural integrity, and simplified and less costly manufacturing compared to prior armatures.
Rotating electrical machines that convert between electrical and mechanical energy are comprised of a rotor portion and a stator portion. In brushless machines, which are preferred for long term reliable operation, stationary armature coils interact with a rotating magnetic field from the rotor. Voltage is induced in armature coils as the rotor rotates, thereby generating power. Likewise, synchronized power can be applied to the armature coils to cause the rotor to rotate, thereby acting as a motor. The armature coils are the key to the conversion between electrical and mechanical energy.
Armature coils have conventionally been wound into slots of lamination stacks in the stator. The slots have served several functions that include providing a low reluctance magnetic path, providing mechanical reinforcement of the coils for transfer of torque and providing an efficient path for conducting heat from the coils. Unfortunately, the use of slotted lamination stacks also has numerous deficiencies, such as magnetic losses caused by variations in the total flux in the electrical machine during rotation. This is particularly important in electrical machines that operate at high power levels, electrical machines that employ unlaminated structures in the field flux path or in applications requiring high efficiency. In addition, these losses cause heating that can shorten operating life. The slotted lamination stacks also increase the inductance of the armature coils, which lowers the power capability, reduces high-speed performance and makes electronic control more difficult. Furthermore, the use of slot winding results in cogging and the slotted configuration reduces the allowable area for placement of armature windings.
To avoid these problems, armatures can be wound as airgap or air core coils. The coils are wound without ferromagnetic cores. The coils are then placed in the magnetic air gap between the rotor and stator. The wound armature windings are can be assembled and bonded together to achieve reinforcement for transfer of torque and to conduct heat from the coils. Unfortunately, winding the armature coils and assembling for later bonding into the stator is difficult and expensive. The bonding to the stator makes such armatures not replaceable and this method is also not possible for use in electrical machines in which both sides of the airgap rotate.
An alternative method for airgap armatures is to wind armature coils and bond them into a reinforced cartridge that is inserted between the rotor and stator. The cartridge can provide the structural rigidity to prevent contact with the rotor and to transfer torque to the stator. To date though, designs and fabrication methods of these armatures have had significant deficiencies. The cartridges require many steps to fabricate and hence are costly. They require multiple individual pieces for fabrication and processes. Coils are each wound individually and then later assembled together using multiple steps. These multi-coil cartridge constructions generally require an undesirably wide airgap thickness, provide marginal structural integrity and low winding density, and are relatively costly.
The invention provides an improved airgap armature for use in brushless rotary electrical machines. The airgap armature is constructed with multiple phase windings comprised of wires that are wound onto and bonded to a supporting form. The form has two ends, each with features for holding end turns of the multiple phase windings. The multiple phase windings have active lengths and end turns, with the active lengths being located on one side of the form and the end turns located on the opposite side of the form. The active lengths, which are located in the armature airgap of the electrical machine, thereby have a minimized thickness by lying down against the form and the airgap armature can achieve maximum winding density. The end turns are held in place while the form provides an easy method for winding as well as increased structural integrity in the final armature. Although the thickness of the form results in an increased airgap thickness in the active region, the form, in the regions containing the active lengths of the windings, can be made thin, and the benefits of the invention outweigh the increased thickness.
The supporting form is preferably constructed from nonmagnetic and nonconductive material to reduce or eliminate eddy current and hysteresis losses. One preferred material is fiberglass laminate that is readily available as manufactured forms, such as sheets and tubes. Because the winding is done onto the form, no handling or assembling of prewound coils is required, reducing manufacturing steps and costs. After winding, the windings are preferably bonded together and to the form to yield a strong and rigid structure that is capable of transmitting reaction torque to the stator. The form can then be simply bolted or mounted into the electrical machine. Unlike printed circuit windings, the form is wound using wires resulting in much higher power capability, winding density and reduced eddy current losses. Multiple individual wires can be used or multiple strand individually insulated conductor wire for further reduction of eddy current losses if desired.
Because of the high winding density and minimized thickness capability of the airgap armature, it is particularly well suited for use in alternators or inductor alternators. Such machines use current applied to a field coil to generate the magnetic flux that passes through the armature windings. Therefore, the requirement for a thin airgap armature is more critical than for permanent magnet machines in order to reduce the required field coil current. One such preferred configuration of inductor alternator for use with the airgap armature has a rotor constructed from ferromagnetic material with multiple circumferentially spaced protrusions. The protrusions face the airgap armature. A coaxial field coil generates homopolar flux in the protrusions and induces alternating current in the multiple phase windings of the airgap armature as the rotor rotates. The high efficiency and reduced complexity of this construction matches the benefits of the airgap armature of the invention. The airgap armature is also applicable for use in permanent magnet excited electrical machines. In these machines, the invention can reduce the required amount of permanent magnet material or increase the power capability.
The increased structural integrity and rigidity of the airgap armature provided by the invention also makes it particularly well suited for use in electrical machines where both sides of the airgap armature are bounded by surfaces of the rotating rotor. This type of electrical machine can eliminate the need for electrical laminations and therefore can achieve even higher efficiency, improvements of typically 3-4%. The airgap armature is used to transfer all torque between the rotor and stationary housing. For high power electrical machines, this torque can be hundreds of inch-pounds or higher. Another advantage of this type of electrical machine is reduced costs from eliminating the need for expensive laminations.
In one embodiment of the invention, the multiple phase windings are wound as serpentine paths around a circumference that is coaxial with the axis or rotation of the rotor. Use of serpentine windings in the airgap armature can provide numerous benefits, including increased structural integrity and rigidity, easier and lower cost manufacturing, and reduced inductance for higher power capability. The windings are inherently structurally linked together by the end turns around the circumference of the armature and the potting of the armature after winding. Multiple serpentine coils are electrically connected in series or parallel to form a complete phase and multiple phases follow sequentially in circumferential position phase to substantially fill the armature structure with conductors. The windings provide support for high power transfer of torque and a power-wise proportionally small airgap. The use of the serpentine windings can eliminate the need to make electrical connections to multiple coils of a single phase around the circumference.
The features on the ends of the supporting form can be holes, slots (elongated holes), pegs, castellations, or any construction that sufficiently holds the end turns on the opposite side of the form as the active lengths and preferably facilitates easier winding. Such features can be molded into the form or alternatively the form can be machined, waterjet cut or laser cut with the features. When using through holes, the multiple phase windings pass through the through holes while traversing between the active lengths and the end turns. Preferably, only one phase of windings passes through each through hole. When using pegs, the multiple phase windings pass between pegs while traversing between the active lengths and the end turns. A similar open end feature construction can also be achieved using holes that have slots to the ends for easier winding of the wires. In a further embodiment of the invention, the manufacturing rate of the airgap armature can be further increased in open end feature forms. The multiple phase windings are wound on to the form simultaneously by placement of the phase active lengths of different phases in succession around the circumference of the airgap armature wherein the windings placed in end features alternate phases and the winding direction between end features also alternates. This eliminates the need for threading windings and allows for much higher speed winding. The winding can be done by hand or by a winding machine. The windings can alternatively be constructed as coils instead of serpentines if desired. Multiple coils can be connected in series of parallel and series coils could alternatively be wound together consecutively around the circumference.
Unlike some airgap armatures that use conductor tension to hold the placement of multiple pieces, making them only useable for axial gap machines that have radial active lengths, the invention is suitable for use in both axial gap and radial gap electrical machines. When used in radial gap machines, the form can be machined in a tubular shape or alternatively, the form can be machined with end features while being substantially flat for lower cost fabrication. The form is then rolled and secured into a cylindrical shape. It may be made as a light weight cylinder attached to one or more ring structures to accurately hold its circular cross-section. The ring structures can be located outside the active lengths or at the ends of the form so as to not increase the airgap thickness.
The bonding of the windings after winding substantially increases the structural integrity and rigidity of the airgap armature, facilitates heat transfer from the windings, and prevents moisture penetration. The bonding can be by vacuum resin impregnation, and a mold or vacuum bag can be utilized to assure an accurate final shape as well as to squeeze the active lengths flat against the form for minimized airgap thickness and maximum winding density. When using stranded or Litz wire, the windings of multiple phases can be made to occupy 100% of the circumference in the active length area by applying pressure. In one embodiment of the invention, the winding density can be maximized for highest efficiency and power capability by utilizing holding features with a conductor space width, W. The rotor of the electrical machine has a number n of same polarity poles. The armature has a number of phases, N, of multiple phase windings, and the airgap armature has a minimum diameter, d, of the active length of the multiple phase windings. The relationship of these parameters in this embodiment is expressed as follows: W=(xcfx80 d)/(2 n N).
The form can have a planar cross-section or alternatively can be non planar. Examples of non planar cross-section include but are not limited to xe2x80x98Lxe2x80x99 or xe2x80x98Zxe2x80x99 shapes. They can provide increased structural rigidity and also facilitate easier installation in some configurations of electrical machines.
One layer of multiple phase windings can be wound on the form or alternatively multiple phases can be wound to provide increased windings with a larger armature airgap. One preferable method for winding multiple layers on a single form is to wind the first layer with a shorter active length than subsequent layers. Multiple layer armatures can also be constructed by using multiple forms in a single magnetic airgap. In this case, the sides of two forms having the active lengths are preferably arranged to be facing each other. This minimizes the required airgap thickness and can increase structural integrity as well as damage tolerance since the active regions are shielded by the forms.
The invention can be utilized in any electrical machines employing airgap armatures such as motors, generators, alternators, hybrid vehicle drives, etc. It is particularly well suited for use in flywheel energy storage systems especially because of the capability for higher efficiency, low inductance, increased structural integrity and rigidity, maximized winding density per airgap thickness and lower cost improved manufacturability.